1. Field of the Invention
A process is provided for selective catalytic conversion of certain hydrocarbon feedstocks to product rich in para-dialkyl substituted benzenes. The catalyst required in the process comprises a crystalline material having the structure of PSH-3, MCM-22 or MCM-49, or a mixture of said crystalline materials, said crystalline material having been treated with one or more monomeric or polymeric siloxane compounds which decompose to oxide or non-oxide ceramic or solid-state carbon species.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline :structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIB element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIB element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195);zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (U.S. Pat. No. Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe crystalline silicate of varying aluminum and metal content.
U.S. Pat. No. 4,439,409 refers to a composition of matter named PSH-3, useful as a catalyst component in the present invention, and its synthesis. U.S. Pat. No. 5,236,575 teaches MCM-49 crystalline material which is another zeolite useful as a catalyst component in the present invention. A composition of matter appearing to be identical to the PSH-3 of U.S. Pat. No. 4,439,409, but with additional structural components, is taught in European Patent Application 293,032. Crystalline MCM-22 is also useful in the present invention as a catalyst component, and its properties and synthesis are taught in U.S. Pat. No. 4,954,325.
Of the xylene isomers, i.e., ortho-, meta-, and para-xylene, meta-xylene is the least desired product, with para-xylene being the most desired product. Para-xylene is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers, such as "Dacron". Mixtures of xylene isomers, whether alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 wt. % para-xylene in the equilibrium mixture, have previously been separated by expensive superfractionation and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.
The term "shape-selective catalysis" describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., Chen, N.Y., et al., Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as isomerization, disproportionation, alkylation and transalkylation of aromatics are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective alkyl-substituted benzene disproportionation to para-dialkyl-substituted benzene.
A representative para-dialkyl-substituted benzene is para-xylene. The production of para-xylene is typically performed by methylation of toluene or by toluene disproportionation over a catalyst under conversion conditions. Examples include the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Soc. 101, 6783 (1979), and toluene disproportionation, as described by Pines in The Chemistry of Catalytic Hydrocarbon Conversions, 72, Academic Press, NY (1981). Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending upon the degree of selectivity of the catalyst for para-xylene (para-selectivity) and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of xylene produced as a proportion of the feedstock, is also affected by the catalyst and the reaction conditions.
The equilibrium reaction for the conversion of toluene to xylene and benzene proceeds as follows: ##STR1##
Disproportionation and alkylation of aromatics are mechanisms for production of para-dialkyl substituted benzenes, such as, for example, para-xylene. The equilibrium composition of xylene product from early catalytic processes is, as shown above, approximately 24 wt. % para-isomer, 54 wt. % meta-isomer, and 22 wt. % ortho-isomer. These early catalytic processes, as shown above, include disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts, described by Grandio et al. in the Oil and Gas Journal, 69, 48 (1971). Also U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879; and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. One such method is to modify the catalyst by treatment with a "selectivating agent". For example, U.S. Pat. Nos. 5,173,461; 4,950,835; 4,927,979; 4,477,583; 4,283,306; and 4,060,568 disclose specific methods for contacting a catalyst with a selectivating agent containing silicon ("silicon compound").
U.S. Pat. No. 4,548,914 describes another modification method involving impregnating catalysts with oxides that are difficult to reduce, such as those of magnesium, calcium, and/or phosphorus, followed by treatment with water vapor to improve para-selectivity.
European Pat. No. 296,582 describes the modification of aluminosilicate catalysts by impregnating such catalysts with phosphorus-containing compounds and further modifying these catalysts by incorporating metals such as manganese, cobalt, silicon and Group IIA elements. The patent also describes the modification of zeolites with silicon containing compounds.
U.S. Pat. No. 4,097,543 teaches a process for selective toluene disproportionation to yield increased para-xylene utilizing a specific crystalline zeolite catalyst, e.g., ZSM-5, which has undergone prior treatment to deposit a controlled amount of carbon coke thereon.
In addition to the above patents, U.S. Pat. No. 2,904,607 refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metalloaluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically said zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. Nos. 3,751,504 and 3,751,506 describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of, for example, a ZSM-5 zeolite catalyst.
The alkylation of toluene with methanol in the presence of a cation-exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis, 16, 273-280 (1970). The workers reported selective production of para-xylene over the approximate temperature range of 200.degree. C. to 275.degree. C., with the maximum yield of para-xylene in the mixture of xylenes, i.e., about 50 percent of the xylene product mixture, being observed at 225.degree. C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in the production of para- and ortho-xylenes. U.S. Pat. No. 3,965,210 describes alkylation of toluene with methanol in the presence of a crystalline aluminosilicate zeolite, such as ZSM-5, which has been modified by contact with a polymer made up of meta-carborane units connected by siloxane units to selectively yield para-xylene. These latter catalysts have, however, suffered from the serious deficiency of loss of selectivity upon air regeneration. This is attributable to breakage of carbon-silicon bonds upon exposure to the high temperature of regeneration giving rise to isolated clusters of silica on the zeolite surface rather than the extensive surface coverage afforded by the technique described herein.
U.S. Pat. Nos. 4,029,716 and 4,067,920 teach use of a specific catalyst, e.g., ZSM-5 or ZSM-11, which has been pretreated with a particular boron compound to produce para-xylene by alkylating toluene with an olefin. U.S. Pat. No. 4,117,026 teaches selective production of para-dialkyl substituted benzenes over catalyst comprising a large crystal zeolite, e.g., ZSM-5, having certain sorption characteristics.
U.S. Pat. No. 2,722,504 describes a catayst of an activated oxide such as silica gel having a thin layer of a silicone polymer deposited thereon to increase the organophilic character of the contact surface and, as such, seeks to avoid silica deposition.
Crystalline aluminosilicate zeolites, modified by reaction with an organic substituted silane, have been described in U.S. Pat. Nos. 3,682,996 and 3,698,157. The former of these patents describes, as novel compositions of matter, crystalline aluminosilicate esters made by reacting a crystalline aluminosilicate having an available hydrogen atom with an organic silane having a SiH group. The resulting compositions were disclosed as being catalysts useful for hydrocarbon processes, particularly hydrocracking. In the latter of the above patents, the use of ZSM-5 crystalline aluminosilicate zeolite modified by treatment with an organic-radical substituted silane is described, together with the use of such modified zeolite in chromatographic separation of the compounds in a C.sub.8 aromatic feedstock.
U.S. Pat. No. 4,145,315 discloses a method for the production of silica-modified zeolite catalysts which are prepared by contacting the specific zeolite with an organic solvent solution such as hexane, of a silcone fluid, distillation of the hexane, and air calcination of the zeolite residue.
Silica-modified catalysts are shown in U.S. Pat. Nos. 4,379,761; 4,100,219; 4,090,981; and 4,127,616. In each instance the silica modification results from interaction of the zeolite portion of the catalyst with an organic solution comprising a silica source such as a silicone. U.S. Pat. No. 4,465,886 teaches selective conversion of hydrocarbon compounds to product rich in para-dialkyl substituted benzenes over catalyst comprising a zeolite, e.g., ZSM-5, ZSM-5/ZSM-11 intermediate, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, or ZSM-48, having deposited thereon a coating of silica which covers exclusively the external surface of the zeolite.
U.S. Pat. No. 4,088,605 shows altering a crystallization medium to substantially eliminate aluminum during crystallization in order to synthesize a zeolite with a coating of silica.
Catalysts comprising MCM-22 or PSH-3, two crystalline materials useful in the present invention, are taught for use in U.S. Pat. Nos. 4,954,663; 4,962,256; 4,962,257; 4,992,606; and 5,001,295, for aromatic compound alkylation or disproportionation in general. U.S. Pat. No. 5,043,512 teaches converting alkylaromatic compounds by isomerization over catalyst comprising crystalline MCM-22.